Method and apparatus for separating particulates from a gas stream

ABSTRACT

A method and apparatus for separating particulate pollutants having a size range from submicrons to several tens of microns from a gas stream. The invention utilizes micron sized liquid particles or mists to remove the particles from the gas stream. The apparatus includes twin fluid atomizers to produce the micron sized particles and a corrugated plate interceptor to remove the liquid and entrain particulate pollutants from the gas stream.

BACKGROUND OF THE INVENTION

The present invention relates to systems designed to remove pollutantsfrom the air including both particulates and gaseous contaminants. Inmany industrial operations a considerable quantity of contaminants areproduced which must be removed from the air before it can be dischargedto the atmosphere. This is particularly true in the case of miningoperations where considerable dust is produced in the form of very smallparticulate matter.

In the past, it has been the practice to use wet scrubbers for removingthe particulate matter from the air because of their low capital costand their ability to remove both the particulate and gaseouscontaminants. While wet scrubbers are in wide use, they have a majordrawback in that their operation requires considerable energy. Theconventional wet scrubber consists of pressure nozzles that producedroplets in the size range of several hundred microns that areineffective in capturing the respirable-size particulates contained innormal contaminants. It has been the customary practice to use a venturisection in a duct to create sufficient velocity of air flow to break upthe large droplets in the scrubbing liquid into small droplets in therange of from 5 to 100 microns. While the use of a venturi sectioneffectively breaks up the large droplets to provide the 5 to 100 micronsize droplets required for removing respirable-size particles from theair, it is a very inefficient means for producing the required dropletsize. This inefficiency results in an increased cost of operation of thesystem. The pressure differential over the scrubber length for aconventional wet scrubber is 10 to 100 inches of water column while thepressure differential for the device of the present invention is 5 to 10inches of water column. From this comparison it is seen that the majorenergy consumption is in scrubber water atomization.

Another drawback of using wet scrubbers is freezing of the scrubbingwater during severe cold weather. Small drops can be quickly frozen asthey encounter the cold air, rendering them ineffective in capturing thedust particles. Therefore, wet scrubbers are rarely used in coldregions.

Another method that is in common use for removing particulate matterfrom air streams is the conventional baghouse in which cloth filters areused to remove the particulate matter from the flowing air stream. Whilethe baghouse is an effective means for removing the respirable sizeparticles from the air stream, it is expensive to install and requiresconsiderable maintenance during operation. The air flow must be reversedperiodically to remove the particulate matter which is collected on theindividual filters to maintain the air flow through the baghouse.Likewise, provisions must be provided for periodically removing theparticulate matter which is collected in the baghouse. This can involvethe expenditure of considerable money and time. In order to perform thevarious maintenance functions on the baghouse, it is necessary to shutdown the baghouse temporarily while the maintenance is being performed.If it is desired to maintain continuous operation, this necessitates theinstallation of sufficient baghouses so that operations can bemaintained while a certain percentage of the baghouses are shut down forroutine maintenance.

In many installations, the particulate matter filling the baghouse canform an explosive mixture. This is especially true in the case of coaldust or dust from food grains.

SUMMARY OF THE INVENTION

The present invention solves the above problems by utilizing steaminjection followed by twin fluid atomizers in a wet scrubber. Toincrease removal efficiency and prevent freezing, steam is injected atthe inlet of the wet scrubber. Steam injection provides a heating sourceto prevent the water droplets freezing under severe cold weatherconditions and in addition, the condensed steam enlarges particle sizeranging from submicron to 5 microns for more efficient separation.Particles in the size range from submicron to 5 micron are the mostdifficult to separate. Condensation of steam on the surface of theparticles thus enhances the particle removal efficiency. The use of twinfluid atomizers will produce a mist having a droplet size in the rangeof 5 to 100 microns which is required for capturing respirable sizeparticulate matter contained in an air stream. The ability towinterproof the device of the present invention allows it to be used inlocations where baghouses are the only choice.

The present invention minimizes the possibility of explosion bycontacting the respirable size particles with a significantconcentration of water to render the particles non-explosive. The systemis designed so that the total air flow is forced through the condensedsteam containing fine particulates and the mist produced by theatomizers to remove substantially all the particulate matter from theair stream. This is accomplished by providing a reduced cross sectionalarea in the duct work of the system so that the mist can completely fillthe cross sectional area of the duct work and thus force all the air toflow through the mist.

The air flow, after passing through the mist, passes through acorrugated plate interceptor where the liquid containing the particulatematter is removed. The liquid removed by the corrugated plateinterceptor flows out of the interceptor and can be removed from theduct work on a continuous basis. Thus, the system will operatecontinuously and will not require routine maintenance such as thatrequired with baghouses. In addition, the system requires less energy tooperate than a wet scrubber.

The use of twin fluid atomizers to produce a mist having droplets of thecorrect size saves the energy required to force the air flow through theventuri section in a wet scrubber. While the use of the venturi sectionreduces the size of the droplets in a wet scrubber, it does not providethe uniform small droplet size produced by twin fluid atomizers. Thus,the removal efficiency of a wet scrubber is much less than the removalefficiency of the scrubber of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more easily understood from the followingdescription when taken in conjunction with the attached drawings inwhich:

FIG. 1 is an elevation view of a system constructed according to thisinvention shown partially in section.

FIG. 2 is a cross section taken along line 2--2 of FIG. 1.

DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to FIGS. 1 and 2, there is shown a duct work 10 forconveying the dust-laden air through the system of the invention.Obviously, the duct work in the system must be designed to accommodatethe total airflow of the system. When extremelly large volumes of airmust be handled, it will be more economical to provide parallelscrubbers instead of a single extremely large system. The duct work isprovided with an inlet 11, a reduced cross sectional area 12 and adischarge outlet 13. The reduced cross sectional area 12 has thecharacteristics of a venturi but its purpose is to reduce the crosssectional area of the air flow and decrease the quantity of mistrequired to fill the complete cross section of the duct work. Since therequired quantity of mist is reduced, the number of twin fluid atomizersand the required volume of liquid will also be reduced.

A series of steam jets 14 are mounted on a manifold 15 disposed at theinlet 11 of the duct work. The manifold is connected to a source of drysteam by the line 16. The steam jets heat the particulate-laden air to atemperature above freezing to prevent freezing of the water mistintroduced by the twin fluid atomizers. The steam, in addition, improvesthe efficiency of the scrubber by condensing on the particles andincreasing their size. The larger size particles are easier to removefrom the air stream and the efficiency of the scrubber is improved.While the use of dry steam is preferred, wet steam can also be used.

The invention utilizes a series of twin fluid atomizers 23 which aremounted on a manifold 21. The manifold 21 is provided with a centraltube 22 which supplies one of the fluids to the atomizer while theannular space surrounding the tube 22 can be used for supplying thesecond fluid to the atomizers. In most installations, it will bepreferable to use water as the liquid and compressed air as the secondfluid for atomizing the liquid. While these two fluids are preferred,other fluids can be used when it is desired to remove other contaminantsfrom an air stream.

As shown in the drawings the atomizers are positioned so that theirdischarge will completely fill the width and height of the reduced crosssectional area 12 of the duct work. While only a single row of atomizersare shown, obviously more than one row may be used for larger size ductwork. The reduced cross sectional area 12 discharges into a parallelplate interceptor 30. In particular, the interceptor shown is acorrugated plate interceptor which contains a large number of parallelcorrugated plates 31. The air flow in passing through the corrugatedplates will cause the liquid containing the particulate matter toimpinge on the plates and be removed from the air flow. The liquid andparticulate matter will flow by gravity to the bottom of theinterceptor. The liquid and particulate matter can then be removed fromthe interceptor through a drain 32 to a disposal location.

In addition, a mesh or screen or fine filter 40 can be added to the tailend of the scrubber as necessary. This could be water washed by a seriesof water jets 41 mounted on a manifold 42. The water jets can becontrolled by a timer or by a pressure differential switch (not shown).

In some mining operations, it may be feasible to add the liquidcontaining the particulates removed from the air stream by the parallelplate interceptor 30 directly back to the mined coal. This isparticularly feasible in cases where the coal is crushed and the systemis used for removing the dust created in the crushing and transportingoperations before the air is discharged to the atmosphere.

If it is not feasible to add the liquid containing the particulates tothe coal stream, it can be disposed of in suitable settling basins wherethe particulate matter will settle out and can be removed.

The slurry could also be pumped away to a remote location withoutallowing for settling.

The present invention provides a highly efficient system for removingrespirable particles from an air stream before it is discharged to theatmosphere. The term `respirable` is used to describe particulates inthe size of submicron to 15 microns. To remove this size particle ofparticulate from an air stream it is necessary that the liquid mist havea droplet size in the range of 5 to 100 microns. In a particularinstallation handling 6000 cubic feet per minute of dust laden air, theinlet had a cross section of 12 by 36 inches. The required mist wasproduced by using five twin-fluid atomizers having a flow rate of 3gallons of water per minute and requiring 65 cubic feet of compressedair at a pressure of 45 psi. The reduced cross sectional area of theduct work had a cross section of 8 inches height and 36 inches widthwhile the corrugated plate interceptor had a total cross sectional areaof 4.4 square feet. This system was effective in removing more than99.5% of the particulates in a range of 0.5 to 15 microns. This removalefficiency was obtained while having an air speed of 50 feet per secondthrough the reduced cross sectional area of the duct work.

In contrast, a wet scrubber designed to provide the same removalefficiency of respirable particles would require ten times more powerconsumption than the use of a twin fluid atomizer. For a high energyscrubber (i.e., 100 in-Hw column pressure drop), the energy consumptionis 15 HP/1000 actual cubic feet per minute or a total of 90 HP/6000actual cubic feet per minute air flow. In contrast, the presentinvention requires only the power to supply 65 cubic feet per minute ofcompressed air at 45 psig, or approximately 7.2 HP. This assumes thatthe energy to move the air through the system and to supply the water isthe same in both systems.

What is claimed is:
 1. A method for separating respiratory size dustparticles from an air stream, comprising:directing the air streamcontaining the respiratory size dust particles along a confined path;producing a water mist by using twin fluid atomizers wherein the dropsforming the mist have a size range of between one and fifty microns,said mist being directed to cover the entire cross section of theconfined path; directing all of said air through a section of theconfined path having a reduced cross section so that all of said airstream passes through said mist; and separating the water containing thedust particles from said air after said air has passed through saidmist.
 2. The method of claim 1 and in addition, injecting a heatingmedium into the air stream prior to directing the air stream through thewater mist.
 3. An apparatus for removing respiratory size particulatepollutants from a gas stream, comprising:duct means disposed to form aconfined flow path for said gas stream; twin fluid atomizing meansdisposed in said duct means for producing a mist of liquid wherein theliquid drops have a size range between one and fifty microns, saidatomizing means being directed so that the mist covers the entire crosssection of the duct, said duct means having a reduced cross sectiondownstream of said atomizing means for causing substantially all of saidgas stream to pass through said mist; and means for separating saidliquid containing the particulate pollutants from said gas stream priorto said gas stream's exit from said duct means.
 4. The apparatus ofclaim 3 and in addition, at least one jet means positioned upstream ofthe atomizing means for injecting a heating medium into the gas stream.5. The apparatus of claim 3 and in addition, a filter means positionedin the duct means downstream of the means for separating the liquid fromthe gas stream.